1. Field of the Invention
The present invention relates to a method for fabricating an electric circuit device, and to a semiconductor device and an electric circuit device, more specifically, to a method for fabricating an electric circuit device including a semiconductor device having a package form of a small size and high density and mounted on a mounting board, and to a semiconductor device involved in the method, and an electric circuit device obtained with the same method.
2. Descriptions of the Related Art
While requirements become stronger and stronger for reducing sizes, thicknesses and weights of digital video cameras, digital cellular telephones, or portable personal computers, or other portable electric apparatuses, recent VLSI semiconductor devices have been reduced by 30% in size during three years to respond to these requirements. On the other hand, it has been studied and developed as an important subject how the packing density of parts on a mounting board can be increased.
Conventionally, as package forms of semiconductor devices, use has been made of the through-hole mounting type (THD: Through Hole Mount Device) such as DIP (Dual In-line Package) or PGA (Pin Grid Array), etc., which involve mounting by inserting lead wires into through-holes provided on a printed wiring board, or the surface mounting type (SMD: Surface Mount Device) such as QFP (Quad Flat Package) or TCP (Tape Carrier Package), etc., which involve mounting by soldering lead wires onto the surface of a printed wiring board.
Moreover, for reducing size and increasing device density, the device package form has been shifted to so-called Chip Size Package (CSP, also called as FBGA (Fine-Pitch BGA)) whose package size is gradually approaching the size of a semiconductor chip.
For realizing devices further decreased in size and further increased in density, a method of mounting bare chips on a mounting board has been developed.
In the bare chip assembly technique, there are a COB form (Chip On Board) which involves die bonding a semiconductor chip on a mounting board with its electrode forming surface to be the upper surface, and electrically connecting electrodes of the semiconductor chip with electrodes of the mounting board by wire bonding, and a flip chip form which involves forming bumps (projecting electrode) on the chip electrodes in advance, and electrically and mechanically connecting the electrodes of the semiconductor chip with the electrodes of the mounting board by the bumps in a face-down manner with the bump forming surface facing the mounting board.
In the flip chip form, the method of forming bumps on a semiconductor chip is roughly divided into a stud bump method using gold or others and solder bump method. Stud bumps are formed on the semiconductor chip by wire bonding using gold or other wires, on the other hand, solder bumps are formed by plating, vapor deposition, ball transfer and so on.
The stud bump method is mainly applied to semiconductor chips in each of which pads are formed near the periphery of the circuit pattern so as to surround the periphery, so-called peripheral pad semiconductor chips, while, in addition to the above peripheral pad semiconductor chips, the solder bump method is widely applied to semiconductor chips including those each having pads arranged in the entire chip area.
A method has been developed for forming bump bonding by means of applying ultrasonic vibration to a semiconductor chip or a mounting board while pressing the semiconductor chip that has bumps formed by the stud bump method or the solder bump method against the mounting board so that the bumps and the electrodes of the mounting board are in close contact.
The above mounting method is explained below with reference to the accompanying figures.
FIG. 1A is a sectional view of a semiconductor device mounted by the above mounting method, while FIG. 1B is a plan view of the same.
To surround the periphery of the electric circuit pattern of a semiconductor chip 10, pad electrodes 11 are formed of for example aluminum or others near the periphery so as to be connected with the electric circuit pattern.
The forming surface of pad electrodes 11 is covered by a not shown surface protection film comprised of for example a silicon nitride film or a polyimide film, leaving openings at places where the pad electrodes 11 are located.
At these openings, bumps 12 are formed of gold or other conductive materials, and are arranged in a square shape.
In this way a peripheral pad semiconductor device 1 is formed.
FIG. 2A is a side view showing the process of mounting of the above semiconductor device 1, while FIG. 2B is a plan view of the core of the same,
In the mounting board 2 on which the semiconductor device 1 is mounted, at the places corresponding to the forming positions of bumps 12 of the semiconductor device 1 that is to be mounted on the upper surface of the board 20 comprised of for example a ceramic-based material, lands 21 (electrode) are formed by means of covering nickel and gold or other platings on pieces of conductive layers comprised of for example copper or others. Not shown printed wires are provided which are connected to the lands 21 and are formed on the front or back surface, or both of the front and back surfaces of the board 20.
To mount the semiconductor device 1, the lands 21 of the mounting board 2 are aligned with the bumps 12 of the semiconductor device 1 and the semiconductor device 1 is placed on the mounting board 2. Onto the upper surface of the semiconductor device 1, a pressure P is imposed through the surface of the projection 3a (bonding tool) of the horn 3. Under the condition that bumps 12 and lands 21 are in close contact, ultrasonic vibration in the DV direction is generated by a vibrator connected to the horn 3. At this time, ultrasonic vibration is imposed by the horn 3 on the places in close contact between bumps 12 and lands 21, while the amplitude thereof is increased by the horn 3.
Here, the direction Dv of ultrasonic vibration is perpendicular to or parallel with the arrangement direction of the bumps 12.
As for the ultrasonic application apparatus, besides the single support type as shown in FIG. 2A, use may also be made of a double support type apparatus as disclosed in Japanese Patent Publication No. 2915350 or Japanese Unexamined Patent Publication (Kokai) No. 11-45912.
In the above description, the horn 3 is preheated for example to 100xc2x0 C., furthermore, as a result of application of ultrasonic vibration, frictional heat is produced at the places in close contact between bumps 12 and lands 21.
In the case that the bumps 12 are formed of gold, and the surfaces of the lands 21 are plated with gold, although a temperature of 209xc2x0 C. or higher is needed to form a bump bonding by metallic bonding between the bumps 12 and the lands 21, as a result of the frictional heat, the temperature at the places in close contact between bumps 12 and lands 21 exceeds the temperature for metallic bonding, so the bumps 12 and lands 21 are able to be connected mechanically and electrically.
In this way, an electric circuit device as shown in FIG. 3 is able to be fabricated in which the semiconductor device 1 is mounted on the mounting board 2.
The electric circuit device with the semiconductor device 1 mounted on the mounting board 2, for example, is sealed for use with resin between the semiconductor device 1 and the mounting board 2.
In the above method of forming bump bonding and mounting by applying ultrasonic vibration, the bump bonding strength can be raised by increasing the amplitude of ultrasonic vibration, increasing the pressure imposed on the semiconductor device, or lengthening the time of application of ultrasonic vibration, and other methods.
However, when the amplitude of ultrasonic vibration is increased, the pressure imposed on the semiconductor device is increased, or the application time of ultrasonic vibration is lengthened to raise the bump bonding strength as described above, there is a danger that crack may occur at the bonding portion of a bump bonding.
Specifically, as shown in FIG. 4, the crack K which takes place at the pad electrodes 11 formed of aluminum or others on the semiconductor chip 10 is called cratering.
Therefore, it is required to adjust the strength of ultrasonic vibration to ensure the bump bonding strength while not causing the cratering or other mechanical damages.
In the related art, however, in semiconductor devices with a number of bumps arranged in any shape, there was a disadvantage that not all bumps can be bonded under the condition that the bonding strength is higher than a desired value and serious mechanical damages do not occur
For example, in the semiconductor chip shown in FIG. 1B, in which bumps 12 are arranged in a line forming a rectangle in the peripheral area of the semiconductor chip, when the direction of ultrasonic vibration is made the same as any side of the rectangle, if the bonding condition is optimized so that the bonding strength of all bumps is higher than a specific value, cratering occurs easily at bumps arranged along the sides perpendicular to the application direction of ultrasonic vibration, especially those bumps at the corners of the rectangle.
On the contrary, if the bonding conditions are controlled so that cratering does not occur for all bumps, sufficient bonding strength cannot be attained for bumps arranged along the sides parallel with the application direction of ultrasonic vibration.
This phenomenon becomes conspicuous especially when boards made of glass-epoxy-based or other materials having low hardness are used as the mounting boards
The present invention is made in consideration of the above disadvantages. Concerning mounting a semiconductor device including a number of bumps arranged in any shape on a mounting board by the ultrasonic bonding, an object of the present invention is to provide a method for fabricating an electric circuit device able to ensure the bonding strength and bond the bumps without occurrence of cratering or other mechanical damages at the locations of all pads in electric connection, and to provide a semiconductor device concerned with the method, and an electric circuit device obtained with the method.
To achieve the above object, according to a first aspect of the present invention, there is provided a method for fabricating an electric circuit device wherein a semiconductor device having a plurality of bumps formed in an approximate polygonal shape or in a straight line arrangement is mounted on a mounting board having electrodes, comprising steps of aligning the bumps with the electrodes and placing the semiconductor device on the mounting board, and while making the bumps and the electrodes contact closely, applying ultrasonic vibration to the semiconductor device in a direction not parallel with any side of the approximate polygon formed by the arranged bumps, or in a direction not parallel with the straight line formed by the arranged bumps.
In the method for fabricating an electric circuit device according to the present invention, preferably, the application direction of ultrasonic vibration on the semiconductor device is such a direction that the dragging force acting on each bump is uniformed during application of ultrasonic vibration, for example, is such a direction that the distance between two bumps along the application direction of ultrasonic vibration is longer than the minimum distance between two bumps.
Furthermore, in the method for fabricating an electric circuit device according to the present invention, preferably, the bumps are arranged in an approximate rectangle on the semiconductor device, and the application direction of ultrasonic vibration on the semiconductor device is the direction of a diagonal of the semiconductor device.
In addition, in the method for fabricating an electric circuit device according to the present invention, preferably, twisted vibration is used as the method of applying ultrasonic vibration to the semiconductor device in a direction not parallel with any side of the approximate polygon formed by the arranged bumps, or in a direction not parallel with the straight line formed by the arranged bumps. Furthermore, preferably, complex vibration including vibration in the normal direction of the mounting board superposed on the twisted vibration is used as the method of applying ultrasonic vibration to the semiconductor device
In addition, to achieve the above object, according to a second aspect of the present invention, there is provided a method for fabricating an electric circuit device in which a semiconductor device having a plurality of bumps formed in an approximate polygonal shape on a semiconductor chip so as to be connected with the circuit pattern of the semiconductor chip is mounted on a mounting board having electrodes, comprising steps of aligning the bumps with the electrodes and placing the semiconductor device on the mounting board, and applying ultrasonic vibration to the semiconductor device while making the bumps and the electrodes contact closely, wherein the bumps at the corners or close to the corners of the approximate polygon formed by the arranged bumps are used as dummy bumps only for mechanical connection.
In addition, to achieve the above object, according to a third aspect of the present invention, there is provided a method for fabricating an electric circuit device in which a semiconductor device having a plurality of bumps formed in an approximate polygonal shape on a semiconductor chip so as to be connected with the circuit pattern of the semiconductor chip is mounted on a mounting board having electrodes, comprising steps of aligning the bumps with the electrodes and placing the semiconductor device on the mounting board, and while making the bumps and the electrodes contact closely, applying ultrasonic vibration to the semiconductor device in a plurality of separate steps in a direction at each step perpendicular to each side of the approximate polygon formed by the arranged bumps.
In addition, to achieve the above object, according to a fourth aspect of the present invention, there is provided a semiconductor device having a plurality of bumps formed in an approximate polygonal shape for electric and mechanical connection with the electrodes on a board, wherein the bumps at the corners or close to the corners of the approximate polygon formed by the arranged bumps are used as dummy bumps only for mechanical connection,
In addition, to achieve the above object, according to a fifth aspect of the present invention, there is provided an electric circuit device in which a semiconductor device having a plurality of bumps formed in an approximate polygonal shape is mounted on a mounting board having a plurality of electrodes in electric and mechanical connection with the bumps of the semiconductor device, and the bumps at the corners or close to the corners of the approximate polygon formed by the arranged bumps are used as dummy bumps only for mechanical connection.